Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si3N4) with lithium yttrium oxide (LiYO2) and zirconia (ZrO2) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si3N4 grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si3N4 grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.